Modulating system



Feb. 14, 1928.

, F. GRAY MODULATING SYSTEM "Filed Oene, .192i

F'\e\d Curren (Mmmmperes) /nvenor Fran/r @my Patented Feb. 14, 1928.

UNITED sTATEs y 1,658,843 PATENT .oFFlcE.

FRANK GRAY, oE NEW YoEx, N. Y., AssI'GNoE 'ro BELL TELEPHONELABORATORIES, INCORPORATED, oE NEW Yoan, N. Y., a conrona'rIoN or NEWvom:

, MODULATING SYSTEM.

the novel combination of a. photoelectric cell with circuits andapparatus whereby, due to 15 the varying characteristics of the cell,light signals directly modulate a carrier signaling current.v

Other features and advantages which are inherent inthe invention whilenot speciii- 20 cally enumerated will be revealed upon a considerationof the description given hereinafter.

Fig. 1 of the drawing shows a photoelectric cell of specialconstruction.

Fig. 2 shows the cell of Fig. 1 employed as a modulator of analternating current.

Fig. 3 illustrates curves showing the relation between the external eldcurrent and the current through the photoelectric cell. l 3 0 Fig. 4 isa cross section of the cell shown `in Fig. 1.

When the light sensitive element of .a photoelectric cell is subjectedto light rays, it is known that negative electrons are emit $5 ted witha velocity which is a function of the potential applied to the anode, orin Vother words this velocity is a function of the strength of fieldbetween the anode and cathode. If these discharged electrons and 40 theions produced by collision are then sub- 'ected to the action of` anexternal magnetic field at right angles to their direction of motion,the number of such electrons or ions reaching the electrodes istherefore mate- 5 rially reduced. Thus,when a photoelectric cell issubjected to a magnetizing field in the proper direction, the value ofthe photoelectric current owing through the cell is determined not onlyby the intensity of illumi- 0 nation of the light sensitive cathode butalso by the strength of the external magnetic field. When the currentfor providing the magnetic field is alternating in character and of afixed amplitude, the output of the 5 photoelectric cell is also acorresponding al- .6, 1925, serial nu. eomaHESSUED ternating current ofthe same frequency but of an amplitude depending upon the intensity ofthe light incident upon the cathode. One convenient form of constructionof a photoelectric cell to be emplo ed accordlng to the resentinvention'ls i ustrated' in Fig. l oft e drawing. This cell comprises aglass vessel 1, either evacuatedor filled with an inert gas, which iscylindrical in shape. Suitably supported-within the vessel are the anodeand cathode 2 and 3, respectively. The cathode 3, as shown in Fig. 4, 1scylindrical and may be of any suitable material, referably metal. Thecathode is coated wit an appropriate substance which under the action oflight emits photoelectrons.. flhe composition of this substance and themanner of coatingthe cathode surfacedo not formfeatures of the presentinvention. Accordlngly, any of the hydrides of the alkali metals vmay beemployed as the light sensitive coating, which may be applied in anywellknown manner; or the cathode may consist of a photo-sensitive metalcoateddirectly onto the glass walls of 3 the vessel. The anode 2 is ametallic rod and is mounted concentrically with regard to the cathode 3.Suitable conductors4 and 5 are sealed-in through the walls of the glassvessel, to provide electrical connections to the electrodes.

Referring to Fig. 2, a descriptionwill now be given of one method ofincorporating a cell, such as shown in Fig. 1, in a signaling circuit tomodulate the signaling current. For this, purpose, the cell 6 issurrounded by a magnetizing coil 7 which is connected in series with thesecondary winding 8' of a transformer 9, and in series with the biasingbattery 11 and the variable resistance v12. The purpose of the battery,11 and the resistance 12 will be described hereinafter. The primarywinding of transformer 9 is connected to a source of alternatingsignaling current 13, represented schematically. The cathode 14 an theanode 15 are connected in series with the' rimary winding 16 of anoutput trans ormer 17 and in series with battery 18 .and variab eresistance 19. The secondary winding of transformer 17 may then beconnected through a suitable amplifying device to the signaling line L.Should the line L be the usual telephone line with lts associatedequipment, the source 13 may be such as to il 'changed by the intensityof generate a carrier current ofI the order of voice frequency. In thiscase, the field winding 7 may be a high impedance, low current winding,and may be provided with an iron core. n

The system disclosed in Fig. 2 1s adjusted in the followin manner:

A redetermlned amount of light, corre- Ispon ing to the maximum exposureof cell 6 under signaling conditions, is allowed to impinge upon thecathode 14 whereby a photoelectric current of say 50 109 amperes 1sproduced, as indicated inxFig. 3. With switch 23 open resistance 12 1sadjusted until the meter 24 registers 1401milliamperes, as indicated bythe abscissa AB of Fig. 3, thus givin a photoelectric current DB asregisterediy meter 26. Switch 23 is then closed and alternating currentis applied to the field winding. It is to be noted that the current fromsource 13 owing through the secondary Winding of transformer 9 1srepresented by th'e curve 1 of Fig. 3, and thev direct current componentflowing through this same winding is represented by the curve CD of Fig.3. The resultant current flowing through the secondary winding oftransformer 9 is therefore represented by the dotted curve 2. It will benoted that this current varies between the limits and 200 milliamperesrespectively. It will also be observed that this current is ofsubstantially the same wave shape as that delivered by the source 1 Thecircuits are now in readiness for the recelption of light signals whichmay be control ed by the varylng transparency of an ex osed film or inany other manner suitab e for signaling by light rays of varyingmtenslty.

Curve 3 of Fig. 3 shows the manner in which the photoelectric currentthrough the cell varies with different values of current through the eld7 and'with a given intensity of light incident upon the cathode 14. Itwill be noted that the portion EF of curve 3 may for all practicalpurposes be consldered as linear in character. Consequently, should afield current represented by curve 2 be applied to the winding 7 theresultant photoelectric current will vary according to the portionEF ofcurve 3. The amplitude of this current may thereupon be light raysincident upon cathode 14.

When the cell is to be employed for modulating a carrier current forlpicture transmission, the picture in the form of a transparent film 28is wrapped around thecylindrical glass drum 29. Drum 29 is suitably'connected to a driving shaft whichnimparts a rotational movement tosaid drum which is mounted in threaded supports to also provide alongitudinal movement. As a result h of this longitudinal and rotationalmovement, the picture film is scanned in a continuous helical path. Thelight from the source 31 passes through the opening in baille plate 32and thence through the picture film and the transparent drum. 'lhislight is then reiected by a suitable reiiecting system represented bymirror 33, and appropriately focused upon the cathode 4 of cell 6. Whenthe picture is to be transmitted, drum 29 is started in rotation and thecathode 14 is variably exposed to light from the source 31 in accordancewith the degrees of transparency of film 28. This varying quantity of 1iht therefore correspondingly modulates t e amplitude of the alternatingcurrent flowing through the transformer 17.

While in the foregoing description the invention has 'been shown asapplied to a picture transmission system, it is understood thatit may beapplied to any system wherein light signals are to be translated intoelectrical slgnals. Accordingly, the invention is to be limited only bythe appended claims.

What is claimed is: l

1. Means for producing photo-electric current of alternatmg charactercomprising in combination a photoelectric cell, means for subjecting thecell to light variations, andmeans for subjecting the cell to anexternal alternatin magnetic field.

2. In a system Vlor modulating an alternating current by light rays, incombination a light sensitive device, means for subjecting the lightsensitive device to light rays to produce photoelectrons and gaseousions,

and means for producing an electromagnetic field exterior to said lightsensitive device by means of the alternating current to be modulated.

l3. In combination, a photo-electric cell, a source of light, means forproducing in said cell under control of light from said source aphoto-electric current with a rectilinear characteristic, a source ofalternating current, and means energized by said alternating current forchanging said linear photoelectric current to a photo-electric currenthaving a wave shape substantially similar to that of said alternatingcurrent.

4.,In combination, a source of light, a photo-electric cell having ananode and a ight sensitive cathode, means including said light sourceand a magnetizing coil around said cell for controlling the amplitude ofthe current through said cell.

5. In combination, a photo-electric cell, and means for setting up amagnetic eld in said cell with a component perpendicular to a pathtakenby electrons in the cell when said field is zero, to control theimpedance of said cell.

6. In combination, a photo-electric cell aving an anode and a cathodeand being f filled with an inert gas, a source of light. for

ions by collision and having a velocity determined by the potential ofsaid anode, and

means external to said cell for producing an alternating magnetic fieldto vary the direction and Velocity of said electrons and said lons.

7. In combination, a photo-electric cell, a source of alternatingsgnalin current, and means for producinga photo-e ectric current throughsald cell of the same wave shape and frequency as said alternatingcurrent, said means vcomprising a magnetizing coil energized by saidalternating current. .e

8. In combination, a photo-electric cell, a source of alternatingsignaling current, and means for producing an alternating photoelectriccurrent through said cell, said means comprising a high impedance fieldcoil external to said cell and adapted to be energized by saidalternating current.

In wltness whereof, I hereunto Subscribe my name this 1st day ofOctober, A. D. 1925.

FRANK GRAY.

